Pitch bonded refractories have been known at least since 1879, being disclosed in U.S. Pat. No. 218,336, Thomas, granted on August 5 of that year. However, the production of such refractories on a large scale in the United States did not commence until about the mid 1950's, when their manufacture for use as linings for basic oxygen furnaces commenced. Additions of a solid carbonaceous material, e.g., carbon black to pitch bonded basic refractories for oxygen vessel use was suggested by Hodnett, Canadian Pat. No. 614,742, granted Feb. 14, 1961, and channel black and fine thermal black as specific carbon black additives were suggested by Wilson, U.S. Pat. No. Re. 27,111, granted Mar. 30, 1971. Montgomery, U.S. Pat. No. 3,468,683, granted Sept. 23, 1969, suggests that pitch bonded refractories wherein the aggregate is dead burned, and is either dolomite or a blend of dolomite with magnesite* can be improved with respect to both hydration resistance and "hot crushing strength" by additions to the batch of elemental sulfur. FNT *The term "dead burned magnesite" is used herein, and in the appended claims, to refer to a dead burned material produced by firing naturally occurring magnesium compounds.
In general, pitch bonded refractories are produced by blending a size graded aggregate and molten pitch to form a batch, introducing a predetermined volume or weight of the batch into a mold, and pressing the batch into a brick. It is common practice for the brick manufacturer to "temper" pitch bonded refractories by heating them to a comparatively low temperature, e.g., 500.degree. F., for several hours. This tempering causes distillation of the more volatile components of the pitch binder, and may cause polymerization of some of the constituents of the binder which are not distilled. In any event, the average softening point of the pitch binder of the refractories is increased by the tempering step.
When pitch bonded refractories are used in basic oxygen steel making, a basic aggregate is required, and they are commonly called pitch bonded basic refractories. In such service they are used to line enormous bottle-shaped vessels to which it is common to charge as much as 300 tons of hot metal from a blast furnace and scrap, as well as substantial quantities of slag-forming ingredients. The charge is then refined, e.g., by blasting the molten surface with a stream of pure oxygen traveling at supersonic speed or by introducing oxygen into the melt through tuyeres in the bottom of the vessel. Before its first "heat" or refining operation, a new lining is first "burned in" by combusting a fuel inside the vessel to heat the lining to a comparatively high temperature. This burning-in causes distillation of some of the lower boiling constituents of the binder, may cause polymerization of other constituents, and ultimately causes carbonization of the remaining binder constituents and the formation of a carbon bond adjacent to the hot face of the refractories in the lining. It is this carbon bond which gives the lining the strength which is necessary for it to withstand the stresses to which it is subjected during the steel making operation. As the vessel is used, and the lining wears away, the region in which the binder is carbonized advances gradually toward the cold face of the vessel lining.
It will be appreciated that the pitch bonded basic refractory lining of an oxygen vessel is particularly vulnerable during burn-in and during the early heats of its campaign. If the softening point of the residual binder is exceeded prior to carbonization for any given brick, that brick, as a practical matter, has no binder and even a slight stress, for example that to which it is subjected by adjacent bricks, can cause large fragments of the brick to break away; this phenomenon is called spalling. It has been observed that spalling of pitch bonded basic refractory linings in oxygen steel making vessels is more likely to occur during burn-in and during the early heats of its campaign. The vulnerability of brick during this part of the campaign, as described above, is believed to be responsible for the observed excessive spalling.